Its impossible to say which is best simply because in some areas one model wins and in others not.
All you can really go on is individual card reviews and the manufacturers reputation. Sapphire, MSI, Gigabyte and ASUS are the ones with the best reputation.
I dont like XFX because they often use reference design cooling or slightly modified reference. That and they voltage lock a lot of their cards, so you have limited overclocking headroom. However they are often cheaper and have a lifetime warranty.
I like Sapphire because they are comparatively cheap among the the aftermarket models and are good quality. They also seem to be one of the few manufacturers that push the envolope when it comes to design. They were the first with a 6GB 7970 and they made it with one of the best coolers on the market (Vapor-X). The only other manufacturer totry was ASUS who ended up making a triple slot card (so it was hard to Crossfire/SLI) and had no advantages in either cooling or noise levels.
Gigabyte are fairly good, they make great but not exceptional cards at a decent price.
MSI are the cream of the crop, they over-design every aspect of their Hawk and Lightning edition cards. If you can afford it, youl be very happy with its performance.